Deposition of polymer at the surface of solid particulate materials can afford polymer/particulate material composites that may be used in various applications. For example, polymer may be deposited to coat and encapsulate the particulate material. In this case, the polymer coating could be used to protect the particulate material from an external environment, to effect the controlled release of the particulate material to an external environment, and/or to alter the surface characteristics that the particulate material presents to an external environment. As a case in point, polymer encapsulated pesticide, herbicide or pharmaceutical particulate materials may be used to provide controlled release products. Alternatively, polymer encapsulated pigment particulate material might be used to enhance certain characteristics of paint formulations.
For the efficiency and reliability of products comprising such polymer/solid particulate material composites, it is generally desirable that polymer is deposited at the surface of the particulate material in a relatively controlled, even and reproducible manner. Where the particulate material is encapsulated in and dispersed throughout a bulk polymeric matrix, it will also be generally desirable that the particulate material is uniformly dispersed throughout that matrix.
To date, a common approach to encapsulate solid particulate materials with polymer has involved dispersing the particulate material into a liquid medium comprising a preformed polymer. The liquid medium may be formed by dissolving the polymer in a solvent, or by simply melting the polymer. Alternatively, the particulate material might be dispersed in monomer which is then polymerised to form the polymer. However, the ability to successfully apply such methodologies is often highly polymer and/or particulate material dependent. Furthermore, dispersing agents (i.e. agents with surface activity such as surfactants) often need to be used to facilitate the dispersion of the particulate material in the coating medium. The use of conventional dispersing agents in this manner can be detrimental to the final products in which the polymer/particulate material composites are employed. In particular, conventional dispersing agents are prone to migrate and localise and thereby undesirably alter the wetting characteristics of the product.
A further problem associated with conventional techniques for depositing polymer to coat and encapsulate solid particulate materials is that they typically afford little if no control over being able to reproducibly coat the particulate material with a relatively uniform layer of polymer at a variety of thicknesses.
As an alternative to coating solid particles with preformed polymer, attempts have been made to use conventional free radical polymerisation processes to form polymer at the surface of the particles. However, such attempts have been generally unsuccessful. In particular, processes employing conventional free radical polymerisation techniques to polymerise monomer at the surface of particulate materials are generally not very controllable and have a tendency to produce pimples of, or uneven, polymer at the surface of the particles rather than a uniform polymer coating.
Numerous other methods for depositing polymer at the surface of solid particulate materials have been reported. However, such methods have generally offered little in the way of improved control over the polymer deposition and/or are commercially non-viable.
Accordingly, there remains a need for a commercially viable method for depositing polymer in a controllable manner at the surface of solid particulate materials. Such a method will preferably be robust, efficient and capable of being applied to a range of polymers and particle types and sizes.